Gordon Brown decided to resign as Prime Minister after Tony Blair told him
that he had lost his mandate to govern, according to Lord Mandelson’s
memoirs.

Despite publicly supporting his bitter rival during the election campaign, Mr Blair turned on Mr Brown as the latter desperately tried to negotiate a deal to remain in power with the Liberal Democrats, claimed the former business secretary.

In his new book The Third Man, Lord Mandelson writes that despite Mr Blair wanting a Left of centre government, he could not countenance the prospect of Mr Brown clinging to power.

“Gordon could not stay … Tony told him and me that the public would simply not accept Gordon remaining,” Lord Mandelson wrote.

Lord Mandelson paints a picture of Mr Brown's increasing desperation to stay in Downing Street despite Labour's defeat in the May 6 election.

The former prime minister refused to be put off by categorical statements by Nick Clegg and other Liberal Democrats that they could not work with him, according to Mr Mandelson.

At one point Mr Brown was said to be travelling between Downing Street and Parliament by underground tunnel and by ordinary car, to avoid the press and the public seeing his attempts to reach a coalition deal.

Mr Brown’s manoeuvres are all the more remarkable given that Mr Clegg had been clear all along that he would not work with the Labour leader, according to Lord Mandelson.

The Lib Dem leader told the then-prime minister: “Please understand, I have no personal animosity whatsoever.

“But it is not possible to secure the legitimacy of a coalition and win a referendum unless you move on in a dignified way.”

Describing the point when negotiations had all but broken down between the Lib Dems and Labour, Lord Mandelson adds: “Nick [Clegg] said the question was whether the very idea of a coalition with Labour was really workable and whether the voters would ever buy it.

“‘The reality,’ he said rather brutally, ‘is that your party is knackered after 13 years in power’”

In a further twist, Lord Mandelson claims that Mr Brown, on his advice, visited the Queen to resign before Mr Clegg had finalised a deal with the Tories, despite Mr Clegg’s protestations.

It placed Mr Clegg in the awkward position of effectively being in a coalition before his party had signed a final agreement.

The move, Lord Mandelson claims, was to avoid Gordon Brown having to leave No. 10 in the dark.

“That was not the image that I wanted for his [Mr Brown’s] leave-taking, nor the one I felt that his lifetime in politics and public service deserved,” Lord Mandelson said.

Lord Mandelson’s decision to publish his memoirs could damage morale, rake up old feuds and distract from Labour’s leadership campaign, MPs and party members have warned.

There are concerns that Lord Mandelson could open old political wounds by publishing his memoirs at such a sensitive time for the party. Tony Blair is also due to publish his autobiography in September.

Peter Kenyon, a member of the party’s ruling National Executive Committee, said the memoirs could be a “distraction” to the leadership campaign.

David Miliband is in the lead in the race at present, ahead of his brother Ed, but members of 150 constituency parties are due to decide who to back over the next fortnight. Unions and MPs will also be choosing in coming weeks.

Mr Kenyon told BBC Radio Five Live that the “party cannot control when these people want to air their prejudices’’.

He added: ‘‘These people like Peter Mandelson and Tony Blair will be out there competing to sell their books. That is what they are now about.”

His concerns were echoed by Hazel Blears, the former communities secretary, who said she was worried that the memoirs could damage morale.

“We’ve done quite well after the election, we lost, we’ve come back, we’ve not been full of recriminations and I think we just need to stay on track,” she said.

Jon Trickett, Mr Brown’s former parliamentary private secretary, added: “It does not serve the party to rake over all this stuff.”

Lord Mandelson’s book has been described by some critics as “Mandy’s hatchet job”. Despite the criticism, some leading members of the party have supported Lord Mandelson’s decision to publish his memoirs.

Lord Prescott, the party’s former deputy leader, told the BBC’s The Andrew Marr Show that Lord Mandelson wanted to write a book “to put the record straight”.

Harriet Harman, the interim leader of the Labour Party, said “memoirs have their place”.